The present invention is concerned with the hydroxymethylation of mixtures containing so-called "non-.alpha.-tocopherols" and the subsequent conversion of the hydroxymethylation products into .alpha.-tocopherol, with the two reaction steps being carried out together as a one-pot process.
As is known, the naturally occurring non-.alpha.-tocopherols, .beta.-, .gamma.- and .delta.-tocopherol, differ from .alpha.-tocopherol, the tocopherol which has the highest vitamin E activity and which is therefore the biologically most valuable tocopherol, by the absence of one or two methyl groups in the 5- and/or 7-position of the chroman part of the molecule. Accordingly, there exists a need for the chemical conversion of such non-.alpha.-tocopherols into (.alpha.-tocopherol, with the main problem residing in the efficient, complete mono- or dimethylation of the benzene ring of the substituted chromanyl group.
Since synthetic processes for the manufacture of .alpha.-tocopherol identical to the natural form thereof have hitherto been found to be uneconomical, and natural, especially vegetable, sources of tocopherols usually contain only a relatively small amount of .alpha.-tocopherol, but predominantly non-.alpha.-tocopherols, such that the isolation of .alpha.-tocopherol from such natural materials (raw materials) is also not particularly viable, the object of the present invention is to develop a process for the conversion of non-.alpha.-tocopherols, which may be present in respective raw materials or obtainable therefrom, into .alpha.-tocopherol, this process being more economical in many respects than previous pertinent processes.
With the same purpose as the object of the present invention, several processes for the conversion of non-.alpha.-tocopherols into .alpha.-tocopherol are known from the state of the art. For example, Japanese Patent Publication (Kokoku) No. 4183/1985 (Eisai Co. Ltd.) discloses a process for the manufacture of .alpha.-tocopherol from non-.alpha.-tocopherols by reacting at least one non-.alpha.-tocopherol with formaldehyde in the presence of boric acid or a derivative thereof under catalytic reducing conditions. This process, which comprises not only hydroxymethylation but also a subsequent reduction, is a one-pot process, so that the isolation of the very unstable hydroxy-methylated intermediates as well as side-reactions are avoided.
However, the process is clearly carried out at relatively high temperatures of about 200.degree. C. (see Examples 1-12), which is known to lead to an undesired decomposition of the formaldehyde to carbon monoxide and hydrogen and consequentially to a pressure increase in the reaction vessel. Further, it is noted that a whole series of allegedly suitable solvents, inter alia also the reagent "alkyl borate" (trialkyl borate, e.g., trimethyl borate), is mentioned; but the possibility of using a combination of solvents is not suggested. In view of the use of relatively high temperatures in the known process, this necessitates stronger and consequently more expensive process equipment (autoclaves) as well as high energy costs for the heating and cooling. In particular, the pressure increase caused by the decomposition of the formaldehyde gives rise to problems, not least dangers, in carrying out the reaction: the gases thereby generated necessitate expensive safety measures, which reduces the economy and renders additional investments necessary.
Further, the best results are achieved when trimethyl borate is used as the single solvent. This use, however, is expensive in the production or recycling and accordingly represents an expensive measure. Therefore, the process disclosed in Japanese Patent Publication No. 4183/1985 has serious disadvantages with respect to plant investment and operating costs.